Don't worry, STC won't switch to cuneiform without warning. What vaguely resembles a chemical formula or a cuneiform text is meant to keep the title as short as possible. More verbose: I will take a closer look at Y-shaped bacteria to see if their "irregular" shape points to the mode of cell division, maybe longitudinal or lengthwise division? Thus, in short: I → Y → II ?
What is an "irregular shape" in bacteria anyway? According to textbooks (and Wikipedia), it's anything that is not rod-shaped, or sphere-shaped (coccoid), or spiral-shaped. (I have a certain fondness for the accepted and widely used but somehow indecisive shape category "coccobacillus"). How diverse the "landscape of shapes" among the bacteria actually is, was pointed out by Manuel Sánchez in his blog Curiosidades de la Microbiología in a 2017 post "On Shape", which he kindly shared with STC. Manuel had referred to the essay of Kysela et al. (2016), and I take the opportunity to remind you of this outstanding compilation: voilà, Figure 1 (legend at the end of this post).
You won't see any genuinely Y-shaped bacteria in Figure 1, but they have been percolating in the literature for decades. Most often, the term "Y-shaped" is found in the context of Bifidobacterium (Actinomycetota), but also mentioned for Sinorhizobium (class Alphaproteobacteria). I will then turn to Treponema (Spirochaetota), and Spiroplasma (Mycoplasmatota) in the second part. Note that after the genus/species names in parentheses I have given the new phylum/class names that microbiologists are expected to get used to, if grudgingly (see Roberto's recent post).
Bifidobacterium bifidum began its journey through taxonomy in 1899 as "Bacillus bifidus", based on its signature Y‑shaped or "bifid" morphology (Latin bifidus, from bi- + -fidus (findere split). Since it was regularly found in the feces of breast-fed infants together with lactobacilli, it was later re-named "Lactobacillus bifidus" but it is now undisputed that B. bifidum and the other 100+ described species of the genus Bifidobacterium belong to the Actinobacteria (Actinomycetota). You see on the frontispiece a representative "portrait" of Bifidobacterium longum, a favorite of probiotics proponents, in Figure 2 a whole bunch of them with many Y‑shaped cells, and here a portrait of B. adolescentis. Note that in the images shown here, but also in others found in the literature, the Y-shaped Bifidobacterium cells are always terminal in filaments.
Dhanashree et al. (2017) studied strains of eight Bifidobacterium species by scanning electron microscopy (SEM) for a possible prevalence of Y‑shape morphology. They concluded that "the growth medium and different conditions (temperatures, pH, salts) do play a major part in the pleomorphic morphology of the Bifidobacterium. ... Not all Bifidobacterium have a bifid morphology, and only a very few are intrinsically bifid‑shaped, such as B. adolescentis. It is well-demonstrated in this study that the intrinsic morphologies of these bacteria are rod-shaped, and all but a few do not adopt the bifid form, even after exposure to stress conditions."
The Y‑shape of Bifidobacterium sp. therefore does not appear to be an intermediate morphology during the normal cell cycle, and in no way indicates a specific cell division mechanism (a PubMed search for papers on cell division in Bifidobacterium returned no hits). Except that the Y‑shape is apparently some kind of stress response by the bifidos, we don't really know why or how they do it. Also, to the best of my knowledge, it is nowhere described that the Y‑shape of the bifidos could be a first step in mycelium formation like, for example, in Streptomyces: the symmetric "arms" of the Y are always of the same length and very short in all available images.
Alphaproteobacteria of the family Rhizobiaceae like Ensifer meliloti (formerly Sinorhizobium meliloti, formerly Rhizobium meliloti) are free‑living, motile, rod‑shaped soil bacteria that can form "dead end" symbioses with certain plants, for example with Medicago sativa (alfalfa or lucerne). A "dead end" for the bacteria as endosymbionts in root nodules, specialized plant tissue, that after a multi‑step differentiation process end up fixing nitrogen (N2) while being nourished by the host. It is for the final stages (zones III+IV) of this endosymbiosis, when growth and cell divisions have stopped, that unusually elongated and Y‑shaped cells are observed in thin sections of nodules (see here). Compared to the already short "arms" of Bifidobacterium Y‑shape cells, the arms of the Sinorhizobium Y-forms appear even shorter. They resemble the stubs at the ends of crossbones in an old 'Jolly Roger' flag of pirates. Only a small proportion of end-differentiated cells show this Y‑shape, and since these cells no longer proliferate it is not a regular intermediate morphology during the vegetative cell cycle of Sinorhizobium.
An add-on: pleomorphic bacteria
The terms "bifid" and "Y‑shaped" are sometimes used synonymously, and when these terms are mentioned in connection with pleomorphic cells (from Ancient Greek πλέω- (pléō-), more, and -μορφή (morphḗ), form/shape), it is usually an attempt to bring at least some scientific order into the jumble of shapes that one sees ─ usually as a minority fraction ─ in most electron microscopic structural studies of bacteria (and if it's not simply due to maltreatment of the cells during preparation for microscopy). Here is an example.
Brock & Edwards (1970) studied the fine structure of Thermus aquaticus ─ the "provider" of Taq polymerase for the polymerase chain reaction (PCR) and member of the newly‑branded phylum Deinococcota ─ by transmission electron microscopy (TEM) and found that its "...cell division mechanism resembles that of typical gram-negative bacteria." Thermus aquaticus cells usually have a diameter of 0.5─0.8 μm, and a length of 5─10 μm. They have the typical layout of Gram‑negative cells with inner membrane (IM), peptidoglycan layer (PG), and outer membrane (OM). Filaments derived from transversally dividing cells have lengths that in some cases exceed 200 μm, and they often form bundles connected via fused OMs or even fused PG, in addition. However, Brock & Edwards (1970) observed that "T. aquaticus occasionally formed swollen and rather pleomorphic cells. Figure 6 shows a section through a filament which has a distinctly bifid morphology" (see here their Figure 6). Just to repeat: these rare bifid Thermus cells are not a regular intermediate morphology during cell division.
I → Y → II ? A clear "no" for Bifidobacterium bifidum and it's siblings. Also "no" for Sinorhizobium (class Alphaproteobacteria). Stay tuned for the second part where I will look at Y‑shaped cells in Treponema (Spirochaetota), Spiroplasma (Mycoplasmatota).